1. Field of the Invention
The present invention generally relates to a method for patterning crystalline indium tin oxide and, more particularly, to a method for patterning crystalline indium tin oxide using femtosecond laser.
2. Description of the Prior Art
In order to improve the device characteristic of the optoelectronic products such as solar cells and flat-panel displays, the amorphous material such as the transparent conductive oxide has to be transferred by thermal treatment into crystalline material so as to reduce the resistivity and enhance the transparency. Generally, six runs of process (five for pattern transfer and one for thermal treatment) are required to complete the crystalline pattern.
To overcome the problems due to the multi-step and high-cost process, laser machining is used in some processing steps to ablate the undesired portion of the thin films. However, convention long pulse laser results in thermal effects to cause elevated ridges on the edge and defects in the layers below. Even though the precision can be improved by using femtosecond laser, the machining efficiency is reduced because of lowered laser intensity to avoid the thermal effects. The currently available femtosecond laser machining is problematic in that high-precision crystalline pattern cannot be formed with high efficiency because high-speed laser machining using increased laser intensity may bring forth thermal effects to cause elevated ridges on the edge.
In U.S. Pat. No. 6,593,593, Nd:YAG laser is used to ablate the zinc oxide (ZnO) and ITO thin films. As shown in FIG. 1, a glass layer 12, an ITO layer 13 and a ZnO layer 14 are formed on a transparent substrate 11. 1064-nm laser is used to ablate the ZnO layer 14 and the ITO layer 13. However, such laser machining suffers from poor precision and thermal effects to cause elevated ridges on the edge and defects in the layers below. Moreover, precision laser optic system for patterning fine line pitch is costly.
In U.S. Pat. No. 6,448,158, excimer laser is used for thermal annealing. As shown in FIG. 2, a laser source 20 is used to emit a 248-nm excimer laser beam 21. The excimer laser beam 21 passes through a beam homogenizer 22, a mask 23 and a focusing lens 24 to perform machining on an ITO layer 27 on a glass substrate 26 disposed on a movable platform 25. However, in this patent, thermal effects resulting from the long-pulse laser lead to poor patterning precision. Moreover, a mask is needed and the patterning of fine line pitch is not available because the precision is limited by the optic diffraction limits.
Therefore, there is need in providing a method for patterning crystalline indium tin oxide using femtosecond laser to make the most of femtosecond laser machining.